A Digital Micromirror Device (DMD) is a type of micro electro-mechanical systems (MEMS) device. Invented in 1987 at Texas Instruments Incorporated, the DMD is a fast, reflective digital light switch. It can be combined with image processing, memory, a light source, and optics to form a digital light Processing® system capable of projecting large, bright, high-contrast color images.
The DMD is fabricated using CMOS-like processes over a CMOS memory. It has an array of individually addressable mirror elements, each having a mirror that can reflect light in one of a plurality of directions depending on the state of an underlying memory cell. By combining the DMD with a suitable light source and projection optics, the mirror reflects incident light either into or out of the pupil of the projection lens. Thus, the first state of the mirror appears bright and the second state of the mirror appears dark. Gray scale is achieved by binary pulse width modulation of the incident light. Color is achieved by using color filters, either stationary or rotating, in combination with one, two, or three DMD chips.
DMD's may have a variety of designs, with the most popular design being a structure consisting of a mirror that is rigidly connected to an underlying yoke. The yoke is connected by two thin mechanically compliant torsion hinges to support posts that are attached to the underlying substrate. Electrostatic fields that develop between the underlying memory cell and the mirror cause rotation in the positive or negative rotation direction.
The fabrication of the above-described DMD superstructure begins with a completed CMOS memory circuit. Through the use of photoresist layers, the superstructure is formed with alternating layers of aluminum for the address electrode, hinge, yoke, and mirror layers and hardened photoresist for sacrificial layers that form air gaps.
While the DMD superstructures are presently quite popular, they do experience certain problems. For example, one problem that today's DMD superstructures experience is the device failure of the mirror layers. This is particularly problematic because the failure of each mirror layer causes a pixel in an image being displayed by the DMD superstructure to be incorrect. It is believed that the failure of the mirror layers is at least partially a function of metal fatigue that the mirror layers experience, for example at the rotation point thereof.
Accordingly, what is needed in the art is a method for manufacturing a microelectronic device, such as a DMD, that does not experience the drawbacks of the prior art methods.